This invention relates to an improved integrated circuit transconductance stage, and to filters employing such transconductance stages.
The field of this invention is related to monolithically integrated continuous-time filters, but it is not exclusively limited to this field. Reference will be made in the description to the field of monolithically integrated continuous-time filters merely for convenience of illustration.
Many continuous-time integrated filter making techniques have been proposed, and implemented. One of the most promising techniques for high-frequency performance is the class which employ a transconductor stage as the filter base block.
This is true because the voltage/current conversion takes place in open loop within the transconductor. This means that the converter is not limited by the unity gain in frequency of the amplification.
The performance characteristics which are usually expected of a transconductor stage for high-frequency applications are basically the following ones:
a capability to handle a wide bandwidth of signals; PA1 low noise production at output and to the source; PA1 low bias current; PA1 a high transconductance g.sub.m value; and PA1 a gain higher than 50 dB.
Previous to the invention disclosed here, a transconductor stage embodiment which has good performance characteristics approaching the above specifications is disclosed in Italian Patent Application No. MI91A 003018 of SGS-Thomson. (Corresponding U.S. application Ser. No. 07/942,678, filed Sep. 9, 1992, is hereby incorporated by reference.)
The transconductance amplifier is described in the above mentioned application is made in mixed Bipolar/MOS technology and contains an input circuit with a differential cell having two MOS transistors with their sources tied together. This is connected to an output circuit which contains a pair of bipolar transistors having their bases tied together.
Although the above mentioned application has many advantages, it has the disadvantage that high transconductance values cannot be obtained. This means that there is a difficulty is achieving low current drain and good linearity.
Another related transconductance stage is described in the article "MOS Tunable Transconductor" (Electronics Letters, 19th Jun. 1986, Vol. 22, No. 13), which is hereby incorporated by reference. In the article a transconductor is suggested whose input stage has a pair of buffer-connected MOS transistors associated with two more MOS transistors forming the gain stage.
In all of these types of amplifiers, the first two MOS transistors are responsible for most of the distortion.
The other transistor pair have their substrates connected to ground and are, therefore, affected by an unfavorable "Body Effect". Accordingly, their threshold voltage is raised by some 0.5 Volts and narrows the overdrive value (Vgs-Vth) range that can be applied to the transistors for programming the transconductance gm value. The latter known solution has, moreover, a disadvantage in that it can only operate at a supply voltage of about ten volts.
According to innovative teachings disclosed herein, there is provided: An integrated circuit transconductor stage, comprising: first and second circuit branches, each including a bipolar transistor and a field-effect transistor in series, the bipolar transistors having bases thereof connected to provide complementary inputs for the transconductor, and the field-effect transistors having gates thereof connected together; and a bias circuit connected to provide a constant bias voltage to the gates, the bias voltage having a value which keeps the field-effect transistors operating in a substantially ohmic regime.
According to innovative teachings disclosed herein, there is provided: An integrated circuit, comprising: first and second circuit branches, each including a bipolar transistor and a field-effect transistor in series, the bipolar transistors having bases thereof connected to provide complementary inputs for the transconductor, the field-effect transistors having gates thereof connected together, and the field-effect transistors having sources thereof connected together; bias circuitry connected to provide a constant bias voltage to the gates, the bias voltage having a value which keeps the field-effect transistors operating in a substantially ohmic regime; a first current source connected to provide a substantially constant current to the sources; and a load connected to provide two current outputs with equal source impedances, opposite in polarity to the first bias current, separately to the bipolar transistors.
According to innovative teachings disclosed herein, there is provided: An integrated circuit, comprising: first and second circuit branches, each including a bipolar transistor and a field-effect transistor in series, the bipolar transistors having bases thereof connected to provide complementary inputs for the transconductor and connected to be driven by equal bias currents, the field-effect transistors having gates thereof connected together, and the field-effect transistors having sources thereof connected together; and bias circuitry connected to provide a constant bias voltage to the gates, the bias voltage having a value which keeps the field-effect transistors operating in a substantially ohmic regime; a first current source connected to provide a substantially constant first bias current to the sources; a load connected to provide two current outputs with equal source impedances, opposite in polarity to the first bias current, separately to the bipolar transistors.
According to innovative teachings disclosed herein, there is provided: A transconductor stage for high-frequency filters, comprising an input circuit portion having signal inputs, and an output circuit portion, and a pair of field-effect transistors having respective gate and source terminals in common, wherein the output portion comprises a pair of bipolar transistors connected to the field-effect transistors.
According to innovative teachings disclosed herein, there is provided: A transconductor stage for high-frequency filters of a type which comprises an input circuit portion having signal inputs and an output circuit portion, incorporates a pair of field-effect transistors having respective gate and source terminals in common, and has the output portion formed of a pair of bipolar transistors connected to the field-effect transistors.
This invention solves the technical problems listed above. Furthermore, it has other advantages that are illustrated throughout this disclosure.